The present invention generally relates to a noise suppressing apparatus for suppressing noises piled up on signals so as to output a signal whose noises are suppressed, and its adjusting apparatus.
A plural mike method, a maximum likelihood noise estimating method, a spectrum subtracting method and so on are provided for the conventional noise suppressing apparatus. The plural mike method is adapted to multiply a signal from each mike by a constant coefficient so as to effect an addition, with the use of a fact that strength differences among signals and noises to be detected with mikes provided in the different positions are different for each of the mikes, for suppressing the noises. In the rough noise estimating method, the average amplitude and dispersion of the noises for each frequency band are calculated by the observation of the noises so as to decide the threshold value of the noise section judgment. Thereafter, upon the inputting operation of the voice with noises being piled up in it, only the voice section exceeding the threshold value is outputted as the signal output. In the spectrum subtracting method, the noise signal spectrum registered in advance from the spectrum of the input signals is subtracted, and thereafter is converted into the voice signal.
It is also reported that a neutral net is used as in the description by Shin-ichi Tamura, Andreks . Wivel: "Noise suppression by waveform input, output using a neutral network" (Shingaku art process Vol. 87, NO. 351, pp. 33-37, January 1988). FIG. 21 is a block diagram of the conventional noise suppressing apparatus. Reference numeral 10 is an input terminal, reference numeral 20 is a buffer memory for storing the input signal data for time length portion of 5 ms. Reference numeral 30 is a four-layer neutral network. Reference numeral 40 is an input layer, reference numeral is a hidden layer, reference numeral 60 is an output layer. Reference numeral 70 is a buffer memory for storing, retaining the output signals of the neutral network. Reference numeral 80 is an output terminal where the data of the buffer memory are sequentially read, outputted.
In the conventional noise suppressing apparatus constructed as described hereinabove, voice signals with noises being piled upon them are inputted from the input terminal, and are stored in the buffer memory 20 of the time length portion of 5 ms. The respective sample data stored are transferred to the respective units of the input layer 40 of the neutral network 30. The neutral network 30 represents voices with noises of 5 ms length being piled up in them on the voice waveform data of 5 ms length with noises being suppressed so as to output them to the buffer memory 70. The data are sequentially read, and are outputted to the output terminal 80 as the voice waveform data after the noise suppression. The learning (determination of a weight coefficient) of the neutral network is effected by a back propagation so that the voices with noises being piled upon them are inputted to the neutral network, and the square total of the difference between the same voice of the noiseless sound and the output signal may become minimum.
The plural mike method had a problem that voices were suppressed inversely when the positional relationship between the noise source and the voice source changed. The maximum likelihood noise estimating method and the spectrum subtracting method had modulated noises in the voice section left with a problem in the natural degree of the voice after the noise suppression. In a system where the time waveform of the input signal was inputted as it was into the neutral net, there was a problem in that the suppressing effect was deteriorated when the voice time pattern of generating speed and so on changed, because the time waveforms with noises being piled up in them were directly represented in the time waveforms with the noises being suppressed in them.